David can speak idiomatic American English and yet, English, he doesn't know it at all.
A. how to teach
B. when it es to teaching
C. when he teaches
D. however he teaches
网友回答
A. how to teach
(答案→)B. when it es to teaching
C. when he teaches
D. however he teaches
解析：when it es to doing sth.是惯用法，意思是“当提到……的时候，”符合题意，而其它选项在逻辑上与上下文不符。